Normally I'd agreed with you and say that Sl2K means Slip 2 stitches knitwise. However, having read the pattern I think it's a typo and should be SSK because there are 72 (74 76) stitches at then end of stage 1/begining of stage 2 and you have 8 stitches less after working each decrease row (apart from row 15). The 4 K2tog will take out 4 stiches so there needs to be 4 SSK to balance this out.
It's not your fault. It's a combination of a strange abbreviation and no glossary in the pattern.